1. Field of the Invention
This invention relates generally to structures for supporting reflective surfaces, and more particularly, to frame arrangements for supporting reflective surfaces made from a thin film material.
2. Discussion of the Related Art
The use of glass to support a layer of metal has been known for centuries to provide a good reflective surface, i.e., a mirror. However, these glass mirrors are inherently fragile and, therefore, are not always appropriate for use in certain unattended environments, in particular, in public areas such as public and school washrooms, gymnasiums, and locker rooms. These unattended or otherwise public areas are among those which are prone to vandalism, and experience has shown that anything breakable located in these areas will eventually be "tested" by such vandals. For at least this reason, conventional glass mirrors are rarely used in these harsh environments. Not only is the glass easily breakable, the resulting glass fragments and shards from a glass-type mirror may cause serious injury and must be carefully removed as soon as possible. To provide a reflective surface in such harsh environments, non-breakable alternatives to glass mirrors are typically used, such as polished metal plates, as detailed below.
Glass mirrors have also been used in optical systems and devices, such as large-screen, rear-projection type television sets wherein an image is projected against a reflective surface onto a viewing screen. Unfortunately, however, the mirrored glass structures used in these devices are both delicate and heavy, and are prone to breaking during shipment, handling, or even during the manufacture of the optical device. These problems are magnified as the demand for larger televisions increases, requiring a larger mirror. If a glass mirror is used for a large television, the mirror will have to be thick to sufficiently support the large surface area of the glass plate. As the thickness of the glass increases, the quality of the reflected image signal decreases.
Another problem associated with glass mirrors is that during the manufacture of the plate glass used with such mirrors, the glass is susceptible to a variety of localized imperfections and its thickness is often difficult to maintain within an acceptable range. Any imperfection inherited by the glass mirror may cause unpredictable distortion to the image signal.
During the development of these large rear-projection type television sets, polished metal plates contemplated to be were used in place of the glass mirrors to provide a reflective surface which overcame the handling and breakage problems associated with the use of glass in the earlier versions. Although the metal plates proved to be essentially unbreakable and were somewhat lighter, the quality of the reflective surface created by the polished plates was significantly reduced, and an unacceptable portion of the image signal was either absorbed or diffused. The result was a significant loss of image light reaching the viewing screen, making the image appearing on the television screen to appear dark and fuzzy.
Another problem associated with the use of polished plates of metal in place of glass mirrors is that the metal plates are sensitive to changes in temperature and can easily become warped, or otherwise thermally damaged, resulting in a distorted reflected image.
To try to overcome these deficiencies, glassless mirrors have been developed that use a thin reflective film supported by a rigid frame structure. U.S. Pat. No. 3,880,500 discloses a mirror having a thin reflective film stretched over and supported by a rectangular frame. The thin film consists of a plastic sheet with a vacuum-deposited reflective layer of metal on one surface. The metal layer is directed inwardly so that the actual reflective surface lies at the boundary between the metal layer and the plastic film.
Although such thin-film mirrors work generally well providing a low cost, safe, and lightweight mirror, such metalized films have an inherent sensitivity to changes in temperature. Since the film is stretched across a metal frame, both the film and the frame will expand and contract in response to ambient temperature. Owing to the differences in size, shape, thermal characteristics, and material, the frame and film will each react differently to changes in temperature. With conventional frames assemblies, the film will effectively contract and possible tear as the frame expands, or will effectively expand while the frame contracts, at which point, the film will create wrinkles or will otherwise distort. Depending on the size of the frame and the material that the frame is made from, these problems associated with the film contracting or expanding may not be noticed. Thus, for example, for relatively small frames (e.g., those frames under 2'.times.2'), the problems associated with the differences in thermal expansion are negligible.
It is, therefore, an object of the invention to provide a frame-supported metalized film mirror that overcomes the deficiencies of the prior art.
It is another object of the invention to provide such a mirror which includes means for compensating for differences in thermal characteristics between the metalized film and the frame.